The present invention relates generally to optical fibers, and in particular, to optical fibers or other polarization-dependent elements connected with optical fibers.
Optical fibers are quickly replacing copper cable as the transmission medium for communication systems, such as the long-distance and local telephone networks and as interconnects within a computer system. The extremely wide band width of optical fibers allows the optical carrier to be modulated at very high data rates.
In addition, the absorption of optical fibers has been reduced to a point where an optical signal can propagate for hundreds of kilometers on an optical fiber without the need for intermediate amplification or regeneration. However, the combination of long distance and high data rates presents new problems in the field of fiber optic communications.
One such problem relates to the polarization dependent loss (PDL) of an optical signal. Polarization dependent loss is the dependence of insertion loss on the state of polarization (SOP) of an input optical light signal. In fiber optic communication, the state of polarization of the light traveling in a fiber optic line must be taken into account. Since semiconductor laser diodes emit light having a specific polarization direction, the performance of many fiber optic components in the system depends on the SOP of the light at their input.
Typically, in fiber optic communication systems, single-mode fibers are used to carry the optical signals. In a single-mode fiber, the optical energy propagates along the fiber path according to one particular mode (hence the name "single-mode" fiber). The electric vector (E) of the mode of the optical signal is in a particular direction orthogonal to the propagation direction. FIG. 1 illustrates this principle, wherein a cross section of an optical fiber is shown. In the example of FIG. 1, the optical fiber is carrying an optical signal which has an electric field vector (E) that is said to be polarized in the vertical direction. As known to those skilled in the art, there is implicitly a possible second electric field at 90 degrees, a horizontally polarized component that is degenerate with the electric field shown in FIG. 1. That is to say, any polarized wave can be composed of a linear combination of two orthogonal waves. Thus, what is normally a so-called "single-mode" fiber can actually be decomposed into two degenerated modes, constituting the E.sub.x and E.sub.y components of the electric vector E.
It turns out that if the single-mode fiber is not completely circularly symmetric or if it contains stress or inhomogeneities then the phase velocities for the E.sub.x and E.sub.y components of the electric field will be slightly different. Such fibers are called "birefringent".
Birefringent fibers are very common since, as light travels along a long fiber, it will inevitably encounter small imperfections such as bends and inhomogeneities that are not all circularly symmetric and therefore will affect light of the two polarizations E.sub.x and E.sub.y differently. For short distances, such effects would not be noticed, but over many meters or kilometers of fiber that is never completely circular in cross section and never without small inhomogeneities and bends, they become important. The combined effect of the birefringence due to non-ideal circular symmetry and the small discontinuities in the fiber is to produce a situation in which light launched with a particular SOP will, in general, change its SOP gradually along the fiber optic path. FIG. 2 graphically illustrates the change of SOP with respect to the phase difference (.PHI.) between the E.sub.x and E.sub.y components of the optical signal's electric field (E).
In the present state of optical fiber technology, there is no control in long fibers over the distribution of the optical power between the two polarization modes E.sub.x and E.sub.y. As a fiber goes around a bend, the fiber becomes birefringent, and a previously well defined single polarization mode is transformed into a combination of the two polarization modes. Indeed, the transformations between the two modes appear to depend upon uncontrolled environmental factors which change over time. Therefore, the light wave arriving at the receiver is of unknown, uncontrolled, and temporarily varying polarization. The lack of polarization control would present no problem if the fiber optic components of the system were polarization insensitive. However, as stated previously, many optical components exhibit polarization dependent loss (PDL) of the optical signal.
The PDL for the majority of fiber optic components typically ranges from 0.05 to 0.3 Db. As the requirements of the fiber optic system become more stringent with time, accurate PDL measurements for the fiber optic components becomes more important. Unfortunately, most photo-detectors for optical power measurement are also polarization sensitive. The detector response typically has a 0.04 to 0.08 Db associated with a change in SOP. This measurement uncertainty is a serious hurdle for accurate PDL measurements of fiber optic components because the measurement uncertainty due to the detector is of the same order of magnitude as the PDL for the fiber optic components themselves.
In order to determine the PDL for a particular optical device under test, the optical signal in which the PDL occurs must be measured by some type of optical detector. However, the detector is also sensitive to the SOP of the optical signal at its input, meaning that the optical signal will undergo a second PDL attributable to the detector. Thus, when measuring the PDL for a particular optical device under test, the measured PDL value will include a first PDL component attributable to the optical device under test (which is precisely what is desired to be measured) and an uncertainty component attributable to the PDL of the detector. Moreover, this uncertainty component will be of the same order of magnitude as the first PDL component, thereby making it extremely difficult to accurately measure the PDL attributable solely to the optical device under test.
In light of this problem, it is a primary objective of the present invention to provide a technique for improving the measurement accuracy of the PDL attributable to a particular optical device under test.